They probably have all older skills in an archive... Or at least I would imagine so, but who knows with SE...

Item Level would only be part of the problem, really, as you would still have stat reworks that happened with Stormblood which is definitely a compounding issue (e.g., Accuracy -> Direct Hit).

They would honestly be better just reworking all the older raids to bring them up to more modern design, rather than trying to decouple it from modern design so people could feel what it was like way back in the day, when we had things like cross class abilities and TP.